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Principles for Utilization of Seawater in the Flue-Gas Desulfurization Process.

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Dev. Chem. Eng. Mineral Process., 9(3/4),pp.211-218,2OOI.
Principles for Utilization of Seawater in the
Flue-Gas Desulfurization Process
Tong Yao
Shenzhen Energy Environment Engineering Co. Ltd., Gangwan Road,
Nanshan District, Shenzhen, P. R. China
The Flake-Hydro Process (SWFGD)is aflue gas desulfiuization method in which sea
water is used for absorption of sulfir dioxide. Shenzhen West Power Plant Unit 4
(300Mw) imported this technologv fiom Norway and installed SWFGD system. At
present, the operational situation is good. All performance guarantees are secured.
Shenzhen Energy Environmental Engineering Co. Ltd, also takes responsibilityfor
monitoring and study of key techniques.
1. Introduction
The Flake-Hydro Process (SWFGD) is a flue gas desulfurization method in which
seawater is used for absorption of sulfur dioxide. The SWFGD process was first
utilized in an extensive way in aluminum-smelting plants and oil refining factories in
Norway. More than twenty FGD plants of this kind have been built over many years.
In recent years, the utilization of SWFGD process in power plants has gained a rapid
growth. In India, the TATA power plant with a capacity of 500 MW has installed
two FGD systems which can each handle a gas flow of 44.5 m 3 N h In Spain, this
system was installed on units of 2 X 80 MW, and in Indonesia the process was chosen
for 2x670 MW units. In China, the National Environmental Protection Association
(NEPA) and the former National Power Department has approved this process for use
on the Shenzhen West Power Plant (SWPT) Unit 4 with a capacity of 300 MW.
211
Tong Yao
ABB Environmental Norway (ABB) is responsible for the detailed design and supply
of equipment, and the Shenzhen Energy Environmental Engineering Co. Ltd. is
responsible for the construction. On 8 March 1999 the system completed 72-hours
of continuous operation and was handed over to the operations department. All
system performance indexes have fulfilled or surpassed the design guarantee.
According to the StatcPower Company and the NEPA, parallel to the construction
of the FGD plants, monitoring of the impact of the SWFGD effluent on the nearby
marine environment and ecology should also be carried out at the same time. This
study of the impact will provide a scientific foundation for the promotion of the
SWFGD process in coastal areas in China.
So far two background and one
afterwards sample collection and analysis has been completed. In addition, NPEPI
and SWPC have also undertaken the “study of the utilization of SWFGD process in
Coastal Areas in China”. A mimic industrial test system has been built in SWPC in
order to study the key technique of the SWFGD process. On 13 March 1999, the State
Power Company hosted a conference conceming this study, the participating experts
determined that this is an advanced study in terms of its quality and range.
2. Principals of the Process and Utilization
Natural seawater contains a large amount of dissolved salt, of which the major
components are chloride, sulphate and some carbonate. Seawater is alkaline by
nature, natural salinity is 1.2-2.5 mmolll, which makes seawater capable ofremoving
SOz fiom flue gas.
The main system includes the flue gas system, seawater supply/discharge system,
electrical and instrument system. The main process and theory is that the flue gas is
picked up &om the bypass duct downstream of the ID fans, through a precipitator, and
then flows through the booster fan to GGHex. The GGHex cools the gas before it
enters the absorber. inside the absorber the packing ensures good contact between
the gas flowing upward and the coolin$ water flowins downwards leading to the
following chemical reactions [I]:
212
Utilization of Seawater in Flue-Gas Desulfurization Process
SO,&,+ H20eH2S0j
H~SO;-H++HSO;'
HSO+H++SO~~-
H'fiom the above reactions will react further with the carbonate in the seawater:
C032-+H'-HC03-
HCO~+H+-H~CO;
1
C02 t +H2
Absorber effluent with absorbed SO2 present as S032- cannot be discharged
directly back to sea. The effluent flows to the seawater treatment plant by gravity.
Here it is mixed with the cooling water. A large amount of air is introduced into the
and driving out Cot. The
water in the aeration basin making SO? oxidize to S
:
O
treated seawater, with pH and COD meeting the environmental standards, can be
discharged back to sea.
The cleaned gas is returned to the GGHex where it is heated again and discharged
to the air through a stack. The following scheme illustrates the SWFGD process:
Flue Gas
-
Precipitator
Washing Seawater
GasIGas
Heat Exchanger
Absorber
Gas
Seawater
9
Aeration
Air
2.1.1
Discharged
Seawater
Studies and Applications
Application of the SWFGD process in coastal areas has been carried out and studied
for over 30 years. The utilization of this process in power plants has achieved a
rapid growth during these years. From 1980 to 1995, there were altogether 25 sets of
213
Tong Yao
SWFGD system in operation, with a total capacity of 3505 MW, total flue gas flow
of 10,940,000 Nm3/h, and the biggest single system is for a 670 M w unit.
Some successful studies and tests has been done by well-known companies around
the world, among which ABB Norway is a leader considering its experience in the
SWFGD process.
The Flake-Hydro process developed by ABB-Flake and
Norsk-Hydro of Norway has received worldwide recognition. Apart from more than
twenty SWFGD plants around world, all flue gas desulfurization plants in Norway use
this technology.
Other desulfurization companies such as Bischoff (Germany),
MISUBISHI (Japan) and Hoosovens (Holland) all have their own advantages. They
are important for further development and improvement of the SWFGD process.
3.
SWFGD Project of SWPC Unit 4
3.1
The Site
Shenzhen West Power Plant is located in the Mawan Harbor area in Nanyou, the
Southwest tip of Shenzhen Nantou Peninsula. The units of the power plant Phase I
project (2x300 MW) are owned by Mawan Power Co. Ltd., SWPC owned phase I1
of the project (2 X 300 MW). To the west of the power plant is Neilingding Ocean at
the mouth of the Pearl River. Except for the east side, the plant is surrounded by sea.
3.2
Design Data
3.2.1
Coal consumption
The coal is fkom the north part of Shanxi, sulfur content is 0.63%.
T-ECR
B-ECR
3.2.2
tm
126.97 tm
114.47
Flue gas flow
T-ECR
l.lX106 Nm3/h
B-ECR
1.2X lo6 Nm'h
Normal temperature is 123'C ,temperature range is 104- 145*C.
214
Utilization of Seawater in Flue-Gas Desulfirization Process
3.2.3
Seawater
Cooling water from Unit 4 condenser is the absorption liquid. Seawater flow is
12m3/s,temperature at condenser outlet is 27-40%, seawater salinity is 2.3%.
3.3
FGD System Performance Guarantee
Under the design condition:
,wantee
check
SO2 removal efficiency
390%
92-97%
Discharged water pH
36.5
6.6-6.9
Effluent water quality meets National Standards (GB3097-87, category 3)
Outlet flue gas temperature
3.4
270%
75--87%
FGD System in SWPC
FGD system for SWPC Unit 4 includes a flue gas system, SOz absorption system,
seawater supply/discharge and restoration system, elecmcal and instrument system,
and control system.
3.4.1
Flue gas system
The flue gas enters the FGD system from the outlet connecting duct of the ID fans.
The emergency bypass damper is closed in normal operation, it is opened in case the
FGD system is stopped and the FGD system inlet and outlet damper are closed. The
flue gas can then be conveyed to the stack directly, avoiding interference with the
power unit normal operation.
The untreated flue gas goes through the booster fan, and the gas-gas heater (GGH)
in which its temperature is lowered, then enters the absorber. The treated flue gas
fiom the absorber flows to the GGH.
The flue gas in the GGH is reheated above 70
%, then exhausted to the bypass duct and on to the stack.
215
Tong Yao
3.4.2 SO2 absorption system
The absorption of SO2 contained in the flue gas is completed in the absorber. The
flue gas enters the absorber at the bottom and goes up through the packing, where the
seawater flowing downward contacts with flue gas. The clean flue gas goes through
the water removal device, which collects the water drops contained in the flue pas at
the top of the absorber. The seawater is induced fiom the packing upper level. The
absorber is a reinforced-concrete structure and contains packing material.
3.4.3 Seawater supplyldischarge system
Water supply and discharge systems of the FGD and the cooling water system of Unit
4 are one entity. The water source comes directly fiom siphon well of the condenser
outlets of the Unit 4, then the seawater is distributed into the intake sump of the
seawater pump and the seawater restoration system (called the aeration basin). The
seawater fiom the intake sump is pumped to the absorber by the booster pump. The
seawater from the absorber outlet flow into the aeration basin by gravity, and the
qualified seawater is discharged to the sea through the discharge channel of Unit 4.
3.4.4 Seawater restoration system
The seawater restoration system includes an aeration basin and aeration system. The
lean alkaline seawater discharged from the condensers, and acid seawater discharged
from the absorber, hlly mix in the aeration basin. A large amount of compressed air
is blown into the aeration basin by the air blower aeration system with abundant
capacity, the tiny bubbles saturate the seawater with oxygen in the aeration basin, and
the sulfite is oxidized to sulfate. The shift of HCOito CO,(aq), and removal of COz
due to the aeration raises the pH of seawater discharge to 6.5, and hence meets the
requirement of seawater discharge.
216
Utilization of Seawater in Flue-Gas Desulfurization Process
4. Purpose of SWPC Unit 4 Demonstration Project and
Development Prospects
4.1 Advantages of SWFGD Process
Table I . Comparisons of FGD techniques.
LimestonelGypsurn
Sulfur content, Yo
>1 .o
SO1 removal efficiency
1
Spray-dry
Limestone
lintotimace
1-3
<2
<2
>90
80-90
60-85
>90
CdS
1.l-1.2
1.5-2.0
2.0-3.0
Block up
Yes
Yes
Yes
I
% of total investment
rather high
rather high
Residue
I 15-20
I
110-15
no
no
rather high
I
I around 7
17-8
high
rather high
Coverage area
large
rather large small
large
State of residue
wet
dry
no
dry
1
I
Operation expense
rather low
1
lzzi
rather low
The SWFGD project has the following easily identified characteristics:
Uses natural seawater as the absorption liquid, no additives, simple and effective.
Absorption system will not produce any residues, and system availability is high.
Treated seawater after cleaning the flue gas is discharged back to the sea, no extra
equipment needed.
SOz removal efficiency is high and it is environmental fiiendly.
Low investment, low operational cost.
4.2
Purpose of the Demonstration Project
For the implementation of Environmental Protection Laws, the control of SO2
emission by the Chinese government is becoming more stringent. The fist priority is
to reduce the threat of acid rain using the flue gas desulfurization technique.
217
Tong Yao
However, high investment and operational costs have always been an impediment to
the development of the FGD technique in China. Over many years, the original
National Power Department and NEPA dedicated their efforts to finding a FGD
technology that has an availability guarantee, requires low investment and low
operational expenses. The Flake-Hydro process can meet all those requirements, it is
indeed a process that can be promoted in China.
China has a long coastline and the economy of the coastal areas are far more
integrated with the rapid development of industry and large populations, and
environmental protection is very important. The numbers of coal-fired power plants
in coastal areas are growing rapidly, this indicates a vast market for the Flake-Hydro
process in China, and within the whole of south-east Asia.
The construction and operation of the SWFGD plant of SWPC Unit 4 will
certainly lay a solid foundation and accumulate precious experience for the
development and promotion of SWFGD processes in China.
5.
Conclusions
The Flake-Hydro Process (SWFGD) is a flue gas desulfurization method in which
seawater is used for absorption of sulfur dioxide. The operation of the system
installed in Shenzhen West Power Plant Unit 4 (300 MW) is good and all performance
guarantees are secured. The desulphurisation efficiency reaches 92-97% for coal
with a sulfur content of 0.63%. After cleaning the flue gas, treated seawater is
discharged back to the sea, no extra equipment is needed. The investment and
operational costs for this system are low. After 72 hours of continuous operation, the
system was handed over to the Operations Department.
References
1. Men& Q.Z. 1980. Inorganic Chemistry. Beijing Normal University Press.
2. 'Mao, J.X.,and Mao, J.Q.
218
1998. Technology of Clean Coal Combustion, Science Press.
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